1. Field of the Invention
The present invention relates to an actuator that includes a cabinet, an electric motor arranged in the cabinet, the motor having a motor shaft, a transmission connected to the motor shaft, the transmission having an output stage, and an activation element connected to the output stage of the transmission, the activation element being intended to cause movement of an adjustable element in the structure in which the actuator is incorporated.
2. The Prior Art
Actuators driven by a reversible low volt DC motor, typically 24V, are used for adjustable articles of furniture, such as beds, chairs and tables. In a linear actuator, the motor, via a transmission, drives a spindle having a nut fixed against rotation so that the nut can move to and fro on the spindle depending on the direction of rotation of the spindle. The nut has arranged thereon a tubular rod whose free end is provided with an attachment for securing in the structure. Typically, the transmission simply consists of a worm gear, where the worm is provided in extension of the motor shaft and the worm wheel secured directly on the spindle. An example is disclosed in WO 02/29284 to Linak A/S. A special structure of linear actuators is called lifting columns, e.g. intended as table legs. Rotary actuators have so far not been as common as linear actuators in connection with furniture, but a single example based on a specially constructed planetary gear is described in WO 01/17401 to Linak A/S. In the furniture business, the price of the actuators is a decisive factor, which has become particularly pronounced recently. This, of course, has resulted in a development toward simpler structures. In the furniture business in particular, an essential aspect is the level of noise, and with a demand for less expensive actuators, it is even more difficult to meet the requirement for low level noise.
During adjustment of the article of furniture, mechanical noise occurs, such as transmission noise, noise in the suspension of the adjustable element, noise because of vibrations that propagate in the structure, etc. When the most powerful single sources of noise have been remedied, it is a complex noise picture that emerges, it being very difficult, it not impossible, to locate the individual sources of noise.
In case of furniture, the level of noise is a significant factor in the sense that this must be as low as possible. This applies to beds, including hospital beds and sickbeds, as well as armchairs and tables, including desks. In the furniture structure and in the drive unit, various common noise-reducing measures may be taken, such as the use of rubber/plastics suspensions, plastics bushings, lubricants, etc. to counteract the noise, but in spite of this it is still desirable to reduce the level of noise additionally.
Some of the noise originates from the motor, where part of the noise is caused by the structure of the rotor with axis-parallel air gaps between the iron flanges on which the coils are wound. This gives small, but noticeable discontinuities in the magnetic field that cause vibrations which propagate in the structure and cause acoustic noise. To obviate the discontinuity, it has been proposed to twist the armature so that the air gap is not axis-parallel. The phenomenon is described in another connection in U.S. Pat. No. 4,616,151 to General Motors Corporation.
Another part of the noise from the motor is caused by an axial movement of the rotor, which likewise results in vibrations and thereby acoustic noise. Owing to manufacturing tolerances it is difficult to do anything about the problem. The phenomenon is also described in another connection inter alia in U.S. Pat. No. 5,497,039 to General Electric Company and U.S. Pat No. 6,069,422 to Fasco Industries, Inc., both of which propose solutions for fixing the rotor in the axial direction via the magnetic fields of the stator.
Another aspect of the acoustic noise is the transmission noise, part of which originates from the engagement between the gear wheels or more particularly the varying degree of the engagement. In this context, reference is made to WO 01/94732 A1 to VKR Holding A/S, which addresses noise problems in relation to window openers, focus being on noise caused by clearance between the components of the transmission, it being attempted to reduce the noise by controlled engagement between a worm and a worm wheel.
The outlined solutions are expensive and complicated or cannot readily be implemented in the present connection.
The invention is based on an ever increasing wish for reducing the noise level of linear actuators, bearing in mind that this must not lead to increased or noticeably increased costs.